Surface treatments are used conventionally for imparting corrosion resistance to a variety of metal substrates. In particular, zinc phosphate treatments are widely used on the metal substrates used for constructing automobiles. However, this zinc phosphate treatment has a problem in that a sludge is generated as a by-product. As a result, next-generation surface treatments that do not use zinc phosphate are now being sought, and one example is a surface treatment that uses zirconium ions (see Japanese Patent Laid-Open No. 2004-218070).
Metal substrates used for constructing automobiles, which require a high degree of corrosion resistance, are subjected to cationic electrodeposition coating following surface treatment. The reasons for performing this cationic electrodeposition coating include not only the fact that the coating film obtained by cationic electrodeposition coating exhibits superior corrosion resistance, but also the fact that cationic electrodeposition coating enables coating to be performed right into the corners of automobile bodies having a complex shape, so-called “throwing power”.
However, recently it has become apparent that in some cases where cationic electrodeposition coating is performed on a metal substrate that has already undergone an aforementioned surface treatment with zirconium ions, a satisfactory throwing power effect may be unattainable. For example, the throwing power is sometimes unsatisfactory when coating cold-rolled steel sheet. If, in this manner, the throwing power is unsatisfactory during the cationic electrodeposition coating, then an adequate level of corrosion resistance cannot be achieved.